Having recently succeeded in preparing antibodies to the purified estrogen receptor protein (estrophilin) of calf uterus and having shown that these cross-react with estrophilin of reproductive tissues of all mammalian species tested, we are attempting to prepare substantial quantities of the purified receptor, leading, in turn, to sufficient amounts of the antibody to permit a variety of immunochemical studies. These include the purification of the immunoglobulin to yield a monospecific antibody, suitable for labeling to provide a simple radioimmunoassay for estrophilin in mammalian tissues and tumors. We propose to use the antibody linked to a supporting medium as an efficient means for receptor purification and to employ peroxidase or ferritin-labeled antibody for the precise intracellular localization of estrophilin by electron microscopy.